The phrase "a certain percentage" refers to an unknown or unspecified amount that is a portion of something. It can be used in various contexts, such as describing how much of a product has been sold (e.g., "We have sold a certain percentage of our inventory this month"), expressing the level of confidence someone has about a prediction ("I'm confident we will achieve a certain percentage of our goal"), or indicating an unknown amount that is part of a whole ("The company made a certain percentage of its profits from overseas operations"). The phrase can also be used to describe a range, such as "We expect sales to increase by a certain percentage this year."